A method is known that when the positions of communication devices are unknown, a target communication device is located by finding a communication path from the target communication device to a communication device whose position is known. For example, the position of a node is estimated based on the distance between adjacent nodes on the communication path, the distance being measured with the strength or delay of radio waves (see Japanese Laid-open Patent Publication No. 2007-221541). Alternatively, communication paths up to nodes whose positions are known are specified and an approximate position of a node is estimated based on the number of hops over each communication path, a communication-cable distance by one hop, and positional information of the nodes whose positions are known (see Japanese Laid-open Patent Publication No. 2006-229845).
However, there are cases where the position of a communication device cannot be estimated accurately due to the communication environment or the positioning of communication devices.